Semiconductor radiation detector

Details Semiconductor radiation detector Semiconductor radiation detector

2000-08-13

2002-09-24

Hannaher, Constantine (Department: 2878)

Radiant energy

Invisible radiant energy responsive electric signalling

Semiconductor system

C257S452000

Reexamination Certificate

active

06455858

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates generally to radiation imaging, and in particular, to apparatus and method for detection of x-ray and light photons using semiconductor radiation detectors, and to methods for fabrication of such devices. The semiconductor radiation detector may also be referred to as a semiconductor detector, a radiation detector or a detector.
Semiconductor radiation detectors typically have an active volume, which is depleted of free charge carriers, and is used to absorb at least some of the radiation to generate charges. There has been a continuing effort in the development of semiconductor radiation detectors with better sensitivity, higher energy resolution, lower electronic noise and larger active area that can operate at or near room temperature. In many applications, the detectors are also required to provide position or imaging information.
For example, semiconductor radiation detectors have been fabricated through the construction of a planar device that can be fully depleted from a small electrode. U.S. Pat. No. 4,688,067 titled “Carrier Transport and Collection in Fully Depleted Semiconductors by a Combined Action of the Space Charge Field and the Field Due to Electrode Voltages” discloses a fully depletable semiconductor detector, which is often referred to as a drift detector. Similar structures are also disclosed in U.S. Pat. No. 4,837,607 titled “Large Area, Low Capacitance Semiconductor Arrangement” and U.S. Pat. No. 4,885,620 titled “Semiconductor Element.” An example of a drift detector is given in
Large Area Silicon Drift Detectors for X
-
Rays
-
New Results,
Jan S. Iwanczyk et al., IEEE Transactions on Nuclear Science, Vol. 46, No. 3, June 1999.
Semiconductor radiation detectors typically have an entrance window electrode to receive impinging radiation. X-ray and light photon detection efficiency of semiconductor radiation detectors is often limited by a dead layer at the entrance window electrode of the radiation detector, in which photons are absorbed but not detected. One of the major contributors to the dead layer is an undepleted region at the entrance window. The x-ray and light photons may be absorbed in the undepleted region before they reach the depletion (active) region, and the charges generated there often recombine and do not contribute to the output signal. This problem is critically important in the detection of low energy x-ray (<5 keV) and visible photons with wavelengths shorter than 600 nm, which are typically absorbed in a very thin layer of the semiconductor. An example of such application is the combination of a semiconductor radiation detector with a scintillating crystal for gamma-ray detection and spectroscopy.
In conventional semiconductor radiation detectors fabricated on n-type bulk material, the entrance window is typically uniformly doped with p+ impurities. The p+ impurity concentration at the entrance window is generally selected such that the depletion region comes close to the outer surface of the detector, but without actually touching the outer surface. Otherwise, large thermally-generated leakage currents may saturate the signal generated by detected radiation.
In a drift detector, due to the use of two superimposed electric fields, the dead layer at the entrance window is typically thick and non-uniform across the uniformly doped entrance window. Since the electric field magnitude at the entrance window electrode varies with location, the uniformly doped entrance window electrode typically depletes deeper (toward the outer surface of the entrance window) in those regions with higher electric field magnitude, compared to those regions with lower electric field magnitude. For example, the undepleted layer typically is the thinnest above the detector anode where the electric field is the strongest, and is thicker in other regions. In fact, at the periphery of the entrance window electrode, the electric field magnitude can be as low as one tenth of the electric field magnitude in the detector anode region.
FIG. 1
illustrates a typical distribution of thickness of the undepleted region across the uniformly doped entrance window. The thicker undepleted layer at the periphery reduces quantum efficiency for the short wavelength light photons and low energy x-rays.
For best detection results, it is also important to consider coupling between the detector and readout electronics. Semiconductor radiation detectors typically have a low capacitance structure. In order to improve electronic noise performance of the low capacitance detector structures, e.g., as disclosed in U.S. Pat. No. 4,688,067, the total input capacitance (including the detector, input transistor, and parasitic capacitance due to interconnections and support structures) should be kept very small. The traditional approach to minimizing the parasitic capacitance is based on the integration of the input transistor to the detector anode, as shown for example in U.S. Pat. No. 5,424,565 titled “Semiconductor Detector.”
When this approach is used, however, it is often difficult to design and fabricate a suitable transistor that will produce the desired characteristics of high signal-to-noise ratio, low capacitance, high g
m
/C
in
, and low 1/f noise. Integrating a transistor with characteristics similar to those of the best discrete JFET (junction field effect transistor) on a high-resistivity silicon wafer is often difficult since technological processes and requirements for fabricating the JFETs are usually quite different from those of the semiconductor radiation detectors. Thus, the integration of the transistor and the detector anode typically compromises the operating characteristics of both the detector and transistor.
Several discrete JFETs exist that offer low input capacitance (<1 pF) and superb characteristics in terms of g
m
/C
in
(8.1 GHz) and 1/f noise (1.3 nV/Hz at 300K for total noise in the range of 1-100 KHz). These JFETs typically have capacitances measuring in a small fraction of a pico farad, making them suitable for use with low capacitance detectors. In addition, gain and noise characteristics of these highly optimized JFETs may be difficult to replicate in a process carried out under the conditions required for fabrication of low capacitance radiation detectors.
Semiconductor radiation detectors often include an outer guard structure at the perimeter of the detector. The outer guard structure can generally prevent premature breakdown, suppress surface leakage current and reduce electronic noise. Prior art detectors used biased or floating p+ rings as outer guard structures on n-type substrates. Unfortunately, structures of this type are typically sensitive to the surface charge density, Q
f
, at the oxide/semiconductor interface, which can vary significantly from process to process. Q
f
is also typically dependent on the ambient conditions in which the detector operates, such as humidity and gas environment, and can take hours to stabilize after bias is applied. In addition, Q
f
can change by an order of magnitude or more after exposure to ionizing radiation. All this makes the optimization and long-term stability of the floating or biased p+ guard ring structures difficult to achieve.
Therefore, there is a need for a semiconductor radiation detector with a thin, relatively uniform undepleted region across the entrance window for optimized detection of low energy x-ray and light photons. There also is a need to reduce parasitic capacitance and coupling noise when the semiconductor radiation detector is coupled to a transistor. Further, there is a need to suppress surface leakage currents and reduce electronic noise, to do so in a way that is relatively insensitive to the surface charge density Q
f
, which may vary in accordance with environmental conditions.
SUMMARY OF THE INVENTION
In one embodiment of the present invention, a radiation detector is formed on a semiconductor material. The radiation detector has first and second major surfaces and an edge surface. The edg

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